In recent years, there have been strong demands for an optical storage medium, such as an optical disk, with higher density and greater capacity in information storage, to allow storing moving-images with high image quality. Furthermore, there also have been strong demands for an optical pickup that is smaller in size and lighter in weight, to allow the optical disk to be used on-the-move.
In response to the demands for smaller size and lighter weight, a variety of integrated pickup apparatuses have been suggested.
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 11-203707 (published on Jul. 30, 1999)) suggests: a semiconductor integrated light emitting apparatus using a photo-detecting device and a beam splitter, both of which are formed on an integrated circuit substrate; and an optical pickup using the light emitting apparatus. The following describes the semiconductor integrated light emitting apparatus 101 and the optical pickup 120, with reference to FIG. 13.
The optical pickup 120 includes a semiconductor integrated light emitting apparatus 101, a grating 106, a hologram 107, a reflecting mirror 110, and an objective lens 111. The semiconductor integrated light emitting apparatus 101 includes an integrated circuit substrate 103, a semiconductor light emitter 102, photo-detecting device 104, a beam splitter 105, and a light absorbing film 108.
The semiconductor light emitter 102, which is included in the semiconductor integrated light emitting apparatus 101, emits laser light, and the beam splitter 105 transmits a constant proportion of the laser light. A part of the laser light emitted from the light emitter 102 does not transmit through the beam splitter 105 but is reflected toward an opposite side to a side on which the photo-detecting device 104 is formed. The laser light thus reflected is absorbed by the light absorbing film 108, which is formed on a side surface of the beam splitter 105. After having transmitted through the beam splitter 105, the laser light is diffracted by the grating 106. The laser light thus diffracted is bent toward the objective lens 111 by the reflecting mirror 110, and is converted to an optical disk 112 by the objective lens 111. Thereafter, the laser light is reflected by the optical disk 112, and then is reflected by the reflecting mirror 110, whereby the laser light is bent toward the light emitting apparatus 101. Then, the laser light enters the hologram 107, is diffracted, and then enters the beam splitter 105. The beam splitter 105 causes an optical path of the laser light to change so as to cause the laser light to enter the photo-detecting device 104, which is formed on the integrated circuit substrate 103.
The photo-detecting device 104 detects a spot size of the laser light, a change in location of the laser light, and the like. As such, a tracking error signal, a focus error signal, and an information signal that is stored in the optical disk 112 are reproduced. These signals are taken out by using commonly-known methods, respectively.
Patent Document 2 (Japanese Patent Publication No. 3545307 (granted on Apr. 16, 2004)) teaches another conventional method. Specifically, the document suggests: an integrated optical unit employing a hologram element and a beam splitter; and an optical pickup employing the integrated optical unit.
FIG. 14 is a diagram illustrating in detail a structure of an integrated optical unit 201, which is a conventional unit described in Patent Document 2.
The integrated optical unit 201 includes: a semiconductor laser (light source) 205; a quarter-wave plate 208; a glass substrate 232 on which a diffraction grating 206 for three beams and a hologram element 209 are formed; a polarizing beam splitter 207, which is a composite prism; a photo-detecting device 210, which is a photo detector; and a package 231. The package 231 includes a stem 231a. The semiconductor laser 205 and the photo-detecting device 210 are mounted on the stem 231a. 
The semiconductor laser 205 emits a light beam 220 (optical axis center 222). The diffraction grating 206 divides the light beam 220 into a main beam (zero-order diffracted light) and two sub-beams (±first-order diffracted light). The light beam 220 thus divided passes through a surface (PBS surface) 207a of the polarizing beam splitter (PBS) 207, and transmits through the quarter-wave plate 208 toward an optical disk (not illustrated). To avoid complication, illustration of the sub-beams (±first-order diffracted light) is omitted in the figure.
After entering the optical disk, the light beam 220 is reflected by the optical disk. A returned light beam 221 (optical axis centers 223, 224), which is a light beam 220 that is reflected by the optical disk, transmits through the quarter-wave plate 208, is reflected by the PBS surface 207a and a surface 207b of a reflecting mirror, and then enters the hologram element 209. After having entered the hologram element 209, the returned light beam 221 is diffracted so as to be divided into positive first-order diffracted light (optical axis center 225a) and negative first-order diffracted light (optical axis center 225b), and then enters the photo-detecting device 210. To avoid complication, only a light beam of an optical axis center of the returned light beam 221 is illustrated in the figure.
The semiconductor laser 205 emits light (P-polarized light) that is linearly polarized in direction X. After having transmitted through the PBS surface 207a, the light is circularly polarized in the quarter-wave plate 208 and then enters the optical disk. Return-light from the optical disk re-enters the quarter-wave plate 208 so as to be converted into light (S-polarized light) that is linearly polarized in direction Y. Then, the light is reflected by the PBS surface 207a. 
By this way, light emitted from the semiconductor laser 205 is directed to the optical disk almost entirely, both the main-beam and the sub-beams. Furthermore, the returned light beam 221 is directed to the photo-detecting device 210 almost entirely. Therefore, light utilization efficiency is high.
In the semiconductor integrated light emitting apparatus 101 of Patent Document 1, the semiconductor light emitter 102, the photo-detecting device 104, and the beam splitter 105 are integrated on the integrated circuit substrate 103. To detect the tracking error signal, the focus error signal, and the information signal, it is necessary to cause laser light to be incident on a predetermined spot on the photo-detecting device 104, which is divided. Therefore, relative positions of the semiconductor light emitter 102 and the photo-detecting device 104 need to be adjusted highly accurately. In other words, relative positions of the semiconductor light emitter 102 and the photo-detecting device 104 need to be adjusted highly accurately because accuracy of an information signal thus obtained and the like is affected by an incident location where laser light is incident on the photo-detecting device 104.
However, the positional adjustment to cause the laser light to be incident on the photo-detecting device 104 is decided by (i) accuracy in bonding (accuracy in electrically coupling and fixing) the semiconductor light emitter 102 to the integrated circuit substrate 103 and (ii) accuracy in mounting the beam splitter 105, because the photo-detecting device 104 is formed on the integrated circuit substrate 103. Thus, the adjustment is less likely to have been accurately performed. Especially a beam-incident location in direction X on the photo-detecting device 104 is affected by fluctuation in height (direction Z shown in FIG. 13) of a light-emitting point of the semiconductor light emitter 102. Further, a beam-incident location in direction Y on the photo-detecting device 104 is affected by fluctuation in position, in direction Y, where the semiconductor light emitter 102 is bonded (direction Y shown in FIG. 13). The two directions are affected by accuracy (fluctuation) in bonding of the semiconductor light emitter 102. Thus, accurate adjustment has been difficult.
Furthermore, even in the case where the semiconductor light emitter 102 is first bonded to the integrated circuit substrate 103, and then emits laser light to adjust the beam splitter 105, because the semiconductor light emitter 102 is not sealed in a package, there is a high possibility of deterioration in characteristics of the semiconductor light emitter 102 due to external force accidentally applied during the adjustment.
Further, in order to seal the semiconductor light emitter 102 and take out an output from the photo-detecting device 104, the integrated circuit substrate 103 needs to be entirely included in a package. As an optical system or the number of divisions of the photo-detecting device changes due to reduction in size and change in design, it becomes necessary to have specially-designed packages. This causes a problem of increase in costs.
In the integrated optical unit 201 of Patent Document 2, the semiconductor laser 205 and the photo-detecting device 210 (photo detector) are combined. If (i) the semiconductor laser 205 or the photo-detecting device 210 is inaccurately mounted, (ii) the package 231, the stem 231a, and the polarizing beam splitter 207 are inaccurately manufactured, or (iii) the hologram element 209, the semiconductor laser 205, the photo-detecting device 210, and the like are out of a tolerance of respective design values, then the semiconductor laser 205 and the photo-detecting device 210 are deviated from their relative positions. This causes problems that a beam converted to the photo-detecting device 210 is deviated from a division line, and that the beam is deviated from a converged state and thus expands. Concrete examples of the tolerance in relation to the conventional techniques include: a tolerance in thickness (direction Z shown in FIG. 14) of the polarizing beam splitter 207; and a tolerance in thickness (direction Z shown in FIG. 14) of the stem 231a on which the photo-detecting device 210 is mounted.